The amount of water that can be used in an electrolytic zinc plant circuit for washing residues and solids is presently limited by the amount of solution washed from the solids (hereinafter termed wash solution) which can be returned to the said cicuit. "Circuit" is hereinafter defined to be an electrolytic zinc plant circuit which may include effluent treatment and water recovery plants but does not include SZP process plants as hereinafter defined. The wash solution must be returned to the circuit or else the zinc values contained, primarily present in the said wash solution as zinc sulphate, are lost. If an excessive amount of wash solution is returned to the circuit, the water balance is no longer satisfactory. A satisfactoy water balance is one where the amount of water entering the circuit in was solution or from other sources equals in total the amount lost from the circuit. As has been stated by D. M. Liddell ("Handbook of Nonferrous Metallurgy Recovery of the Metals", second edition, McGraw-Hill Book Co., Inc., New York, 1945, page 405 at lines 10-12):
An example of a residue washing circuit for an electrolytic zinc plant is given by G. D. Van Arsdale ("Hydrometallurgy of Base Metals", first edition, McGraw-Hill Book Co., Inc., New York, 1953, page 101).
If residues or products are not washed efficiently to displace entrained dissolved zinc with the amount of water available under conditions where the water balance is satisfactory, then either this inefficiency must be accepted, or alternatively, more water must be eliminated from the cicuit by additional evaporation or solution discard, to enable more wash water to be used.
Additional evaporation requires a large input of heat which is both undesirable and costly.
Two methods are known from the prior art for removing zinc values from solutions to give zinc depleted solutions suitable for discard.
The first method is termed "spent stripping" where portion of the spent electrolyte from the electrowinning step of the electrolytic zinc process is electrolysed further to reduce its zinc content. The resultant solution still has a significant zinc concentration and a high sulphuric acid concentration. The cost of treating this effluent to legislative limits is therefore high.
The second method depends upon precipitating zinc from the aqueous zinc sulphate solution to be discarded as a basic compound which is capable of subsequent use as a neutralizing agent in an electrolytic zinc plant circuit. Australian Pat. No. 429,078 discloses that a basic zinc sulphate can be precipitated selectiely from a solution of aqueous zinc sulphate with a variety of precipitants at temperatures in the range 40.degree. C. to the boiling point of the solution at atmospheric pressure: calcined zinc sulphide concentrate (hereinafter termed calcine) and limestone (calcium carbonate) are among the precipitants specified in the patent. A paper printed in Metallurgical Transactions B published by the American Society for Metals and the Metallurgical Society of A.I.M.E., Volume 11B, March 1980, pages 73-82, discloses that basic zinc sulphate is considered to be precipitated by these two precipitants according to equations of the type EQU ZnSO.sub.4 +3ZnO+7H.sub.2 O.fwdarw.ZnSO.sub.4.3Zn(OH).sub.2.4H.sub.2 O
and
3CaCO.sub.3 +4ZnSO.sub.4 +13H.sub.2 O.fwdarw.ZnSO.sub.4.3Zn(OH).sub.2 4H.sub.2 O+3[CaSO.sub.4.2H.sub.2 O]+3CO.sub.2,
respectively. As disclosed in Australian Pat. No. 429,078 zinc present in an aqueous solution of zinc sulphate can be precipitated under appropriate conditions as a basic zinc sulphate such that the latter has a lower content of undesirable ions. Thus the undesirable ions substantially remain in the treated zinc depleted solution. Accordingly the precipitation of the zinc as a basic zinc sulphate is selective and the process for using this precipitation reaction has been termed theSelective Zinc Precipitation(SZP) process. A number of undesirable ions are defined in the patent, of which magnesium, manganese, chloride, sodium, and potassium are of concern in the case of the present invention. For convenience any of the above undesirable ions are preferred to herein as impurity I. It must be understood that more than one of the above undesirable ions may be present in solutions treated by the SZP process and that the said process is capable of effecting control over each of the said undesirable ions present, although possibly to a differing degree. Thus the term "impurty I" may cover both one or a multiplicity of undesirable ions drawn from the group magnesium, manganese, chloride, sodium, and potassium. The treated zinc depleted solution arising from the SZP process is hereinafter termed SZP solution.
The SZP process is therefore capable of effecting control over both the water balance and the concentration of impurity I in the said circuit. The aforesaid paper describes in some detail two alternative methods of discarding the SZP solution remaining after removal of solids using a solids-liquid separation procedure. The first method is direct discard - see FIG. 1 of the paper. Although the SZP solution so discarded will be depleted in zinc, it will generally have concentrations of zinc and other non-ferrous metals in excess of the legislative limits applying to effluents from electrolytic zinc plants. The discarded solution will then require expensive additional treatment before discharge from the plant site. The second method is indirect discard and is accomplished by using the SZP solution remaining after removal of solids for washing solid residues which are removed from the circuit. Jarosite, goethite, and lead residue treatment process are examples of such solid residues. When such residues are washed by SZP solution, the latter displaces the mother liquor of high zinc concentration associated with the solid residues. Thus when the solid residues leave the circuit they contain significant amounts of impurity I via entrained SZP solution. Accordingly the concentration of impurity I in the said circuit can be effectively controlled. This second method of discard has the particular advantage of avoiding the need to process SZP solution in an effluent treatment plant. It thus satisfies the general aim of achieving zero discharge of effluents from metallurgical or manufacturing processes.
One attractive flowsheet incorporating this mwethod of discard is to decycle the wash solution produced by washing solid residues with SZP solution, back to the SZP process, that is, the wash solution becomes the feed solution to the SZP process. This particular flowsheet has been termed closed circuit washing and is shown in FIG. 2 of the paper.
The closed circuit flowsheet of FIG. 2 has a marked advantage with respect to water balance over the flowsheet of FIG. 1 incorporating direct discard. This is exemplified by examination of Table 1 below, which is a clarified version of Table V of the paper. The size of the SZP process plant is the amount of zinc in feed solution to the SZP process plant expressed as a percentage of the total amount of zinc extracted from calcine. Details of the specific design parameters used are set out in the paper.
Table I shows though that the steady-state magnesium concentration with closed circuit washing (FIG. 2) is higher than that for the flowsheet of FIG. 1, even though both are markedly below that for a circuit with no SZP process plant.
TABLE 1 __________________________________________________________________________ Steady state Net circuit input Type of flowsheet Size of magnesium of water in m.sup.3 / FIG. no. in SZP process Zinc concentration 1000 kg of Description paper plant (percent) precipitant (gram/liter) extracted zinc __________________________________________________________________________ No SZP process plant -- 0 None 53.0 0.195 SZP process plant 1 5 Limestone 11.7 0.055 with direct discard SZP process plant 2 5 Limestone 14.5 -0.071 with closed circuit washing SZP process plant 1 5 Calcine 13.3 0.309 with direct discard SZP process plant 2 5 Calcine 16.0 0.160 with closed circuit washing __________________________________________________________________________
The use of calcine instead of limestone as the precipitant of basic zinc sulphate is attractive as calcine is produced for subsequent leaching in most of the existing electrolytic zinc plant circuits and use of a portion in the SZP process plant does not preclude the subsequent extraction of its contained zinc content. Limestone has to be purchased for use in a SZP process plant and furthermore may remove far more than the desired amount of sulphate from the circuit: any such excess sulphate removal will have to be compensated for by addition of sulphate to the circuit. In most cases sulphuric acid will be the cheapest suitable source of sulphate, but even so its cost will be appreciable. Additionally the gypsum arising from the use of limestone in the SZP process plant will have to be disposed of and this may be difficult in practice. However, the substitution of calcine for limestone has an unacceptable adverse effect upon the water balance, as may be seen for either the flowsheet of FIGS. 1 or 2--see Table 1. This is due to the fact that
One object of the present invention is to provide an improved procedure for substituting calcine for limestone, either wholly or in part, as the precipitant of basic zinc sulphate in the SZP process plant, such that the water balance is acceptable. Another objective is to obtain a steady state concentration of impurity I lower than that when using either limestone or calcine according to procedures of the prior art.